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		Eversion of imaginal discs in response to ecdysone in Drosophila Developmental Biology Laboratory Lauren Fety and Ann Marie Lam, Swarthmore College, Spring 2004 Objective This laboratory exercise examines the effects of varying concentrations of ecdysone on the eversion of Drosophila imaginal discs. Introduction Metamorphosis is a developmental stage characterized by vast changes in tissue and form. In Drosophila, metamorphosis begins at the end of the third instar larval stage approximately 120 hours after fertilization. During metamorphosis, larval tissues are replaced by adult structures derived from imaginal discs. These remain sequestered in small compartments called imaginal discs during larval development. Imaginal discs consist of small epithelial sacs that form from invaginations of the ectoderm during embryogenesis (Gates et al. 2000). As the larva develops, the imaginal discs become spatially patterned along the body axis. There are 9 pairs of imaginal discs (labrum, labial, antennal, eye, wing, three legs, and halteres) and a single disc that gives rise to the gonads (see protocol). The nineteen imaginal discs are arranged symmetrically along the body length of the larva and evert, elongate, and differentiate during metamorphosis (Alberts et al. 2002). As the new appendages emerge from the imaginal discs, larval tissues undergo programmed cell death and are eventually replaced by the adult structures (Gilbert 2003). The elongation of the adult structures is accomplished through cell shape changes and is not accompanied by additional cell division (Condic et al. 1990). The unfolding of adult structures from the imaginal discs is signaled by increased levels of the steroid hormone 20-hydroxyecdysone, or ecdysone. A pulse of ecdysone occurs at the end of the third larval instar phase, which signals the beginning of metamorphosis (Riddiford et al. 1993). Another ecdysone pulse occurs about 10-12 hours after puparium formation that triggers pupation and eversion of adult appendages from the imaginal discs (Ward et al. 2003). Ecdysone has been implicated in both triggering programmed cell death in larval tissues and signaling changes in cell shape during imaginal disc eversion (Kozlova et al. 2003). In this experiment, imaginal discs will be removed from Drosophila 3rd instar larvae, exposed to varying concentrations of ecdysone, and observed for disc eversion in vitro.